Semiconductor device and method for manufacturing the same

Abstract

To provide a semiconductor device which has transistor characteristics with little variation and includes an oxide semiconductor. The semiconductor device includes an insulating film over a conductive film and an oxide semiconductor film over the insulating film. The oxide semiconductor film includes a first oxide semiconductor layer, a second oxide semiconductor layer over the first oxide semiconductor layer, and a third oxide semiconductor layer over the second oxide semiconductor layer. The energy level of a bottom of a conduction band of the second oxide semiconductor layer is lower than those of the first and third oxide semiconductor layers. An end portion of the second oxide semiconductor layer is positioned on an inner side than an end portion of the first oxide semiconductor layer.

What is claimed is:  
     
       1. A semiconductor device comprising:
 an insulating film over a conductive film; and 
 an oxide semiconductor film over the insulating film, 
 wherein the oxide semiconductor film includes a first oxide semiconductor layer, a second oxide semiconductor layer over the first oxide semiconductor layer, and a third oxide semiconductor layer over the second oxide semiconductor layer, 
 wherein an energy level of a bottom of a conduction band of the second oxide semiconductor layer is lower than an energy level of a bottom of a conduction band of the first oxide semiconductor layer and an energy level of a bottom of a conduction band of the third oxide semiconductor layer, and 
 wherein an end portion of the second oxide semiconductor layer is positioned on an inner side than an end portion of the first oxide semiconductor layer and a distance between the end portion of the second oxide semiconductor layer and the end portion of the first oxide semiconductor layer is longer than or equal to the sum of thicknesses of the second oxide semiconductor layer and the third oxide semiconductor layer. 
 
     
     
       2. The semiconductor device according to  claim 1 ,
 wherein the first oxide semiconductor layer and the third oxide semiconductor layer comprise at least indium and gallium in an atomic ratio where an indium content is lower than or equal to a gallium content, and 
 wherein the second oxide semiconductor layer comprises at least indium and gallium in an atomic ratio where an indium content is higher than a gallium content. 
 
     
     
       3. The semiconductor device according to  claim 1 ,
 wherein the insulating film has a stacked-layer structure. 
 
     
     
       4. The semiconductor device according to  claim 1 ,
 wherein In a M2 b Zn c O x  (a is a real number greater than or equal to 0 and less than or equal to 2, b is a real number greater than 0 and less than or equal to 5, c is a real number greater than or equal to 0 and less than or equal to 5, and x is an arbitrary real number) is used for the first oxide semiconductor layer, and 
 wherein M2 is Ga, Mg, Hf, Al, Zr, Sn, or lanthanoid. 
 
     
     
       5. The semiconductor device according to  claim 1 ,
 wherein a material represented as In d M5 e Zn f O x  (d is a real number greater than 0 and less than or equal to 5, e is a real number greater than or equal to 0 and less than or equal to 3, f is a real number greater than 0 and less than or equal to 5, and x is an arbitrary positive number) is used for the second oxide semiconductor layer, and 
 wherein M5 is Ga, Mg, Hf, Al, Zr, Sn, or lanthanoid. 
 
     
     
       6. The semiconductor device according to  claim 1 ,
 wherein a material represented as In g M8 h Zn i O x  (g is a real number greater than or equal to 0 and less than or equal to 2, h is a real number greater than 0 and less than or equal to 5, i is a real number greater than or equal to 0 and less than or equal to 5, and x is an arbitrary real number) is used for the third oxide semiconductor layer, and 
 wherein M8 is Ga, Mg, Hf, Al, Zr, Sn, or lanthanoid. 
 
     
     
       7. An electronic device comprising the semiconductor device according to  claim 1 . 
     
     
       8. A semiconductor device comprising:
 an insulating film over a conductive film; and 
 an oxide semiconductor film over the insulating film, 
 wherein the oxide semiconductor film includes a first oxide semiconductor layer, a second oxide semiconductor layer over the first oxide semiconductor layer, and a third oxide semiconductor layer over the second oxide semiconductor layer, 
 wherein an energy level of a bottom of a conduction band of the second oxide semiconductor layer is lower than an energy level of a bottom of a conduction band of the first oxide semiconductor layer and an energy level of a bottom of a conduction band of the third oxide semiconductor layer, 
 wherein an end portion of the second oxide semiconductor layer is positioned on an inner side than an end portion of the first oxide semiconductor layer and a distance between the end portion of the second oxide semiconductor layer and the end portion of the first oxide semiconductor layer is longer than or equal to the sum of thicknesses of the second oxide semiconductor layer and the third oxide semiconductor layer, and 
 wherein the end portion of the first oxide semiconductor layer and the end portion of the second oxide semiconductor layer are in contact with the third oxide semiconductor layer. 
 
     
     
       9. The semiconductor device according to  claim 8 ,
 wherein the first oxide semiconductor layer and the third oxide semiconductor layer comprise at least indium and gallium in an atomic ratio where an indium content is lower than or equal to a gallium content, and 
 wherein the second oxide semiconductor layer comprises at least indium and gallium in an atomic ratio where an indium content is higher than a gallium content. 
 
     
     
       10. The semiconductor device according to  claim 8 ,
 wherein the insulating film has a stacked-layer structure. 
 
     
     
       11. The semiconductor device according to  claim 8 ,
 wherein In a M2 b Zn c O x  (a is a real number greater than or equal to 0 and less than or equal to 2, b is a real number greater than 0 and less than or equal to 5, c is a real number greater than or equal to 0 and less than or equal to 5, and x is an arbitrary real number) is used for the first oxide semiconductor layer, and 
 wherein M2 is Ga, Mg, Hf, Al, Zr, Sn, or lanthanoid. 
 
     
     
       12. The semiconductor device according to  claim 8 ,
 wherein a material represented as In d M5 e Zn f O x  (d is a real number greater than 0 and less than or equal to 5, e is a real number greater than or equal to 0 and less than or equal to 3, f is a real number greater than 0 and less than or equal to 5, and x is an arbitrary positive number) is used for the second oxide semiconductor layer, and 
 wherein M5 is Ga, Mg, Hf, Al, Zr, Sn, or lanthanoid. 
 
     
     
       13. The semiconductor device according to  claim 8 ,
 wherein a material represented as In g M8 h Zn i O x  (g is a real number greater than or equal to 0 and less than or equal to 2, h is a real number greater than 0 and less than or equal to 5, i is a real number greater than or equal to 0 and less than or equal to 5, and x is an arbitrary real number) is used for the third oxide semiconductor layer, and 
 wherein M8 is Ga, Mg, Hf, Al, Zr, Sn, or lanthanoid. 
 
     
     
       14. An electronic device comprising the semiconductor device according to  claim 8 . 
     
     
       15. A semiconductor device comprising:
 an insulating film over a conductive film; and 
 an oxide semiconductor film over the insulating film, 
 wherein the oxide semiconductor film includes a first oxide semiconductor layer, a second oxide semiconductor layer over the first oxide semiconductor layer, and a third oxide semiconductor layer over the second oxide semiconductor layer, 
 wherein an energy level of a bottom of a conduction band of the second oxide semiconductor layer is lower than an energy level of a bottom of a conduction band of the third oxide semiconductor layer, and 
 wherein an end portion of the second oxide semiconductor layer is positioned on an inner side than an end portion of the first oxide semiconductor layer and a distance between the end portion of the second oxide semiconductor layer and the end portion of the first oxide semiconductor layer is longer than or equal to the sum of thicknesses of the second oxide semiconductor layer and the third oxide semiconductor layer. 
 
     
     
       16. The semiconductor device according to  claim 15 ,
 wherein the first oxide semiconductor layer and the third oxide semiconductor layer comprise at least indium and gallium in an atomic ratio where an indium content is lower than or equal to a gallium content, and 
 wherein the second oxide semiconductor layer comprises at least indium and gallium in an atomic ratio where an indium content is higher than a gallium content. 
 
     
     
       17. The semiconductor device according to  claim 15 ,
 wherein the insulating film has a stacked-layer structure. 
 
     
     
       18. The semiconductor device according to  claim 15 ,
 wherein In a M2 b Zn c O x  (a is a real number greater than or equal to 0 and less than or equal to 2, b is a real number greater than 0 and less than or equal to 5, c is a real number greater than or equal to 0 and less than or equal to 5, and x is an arbitrary real number) is used for the first oxide semiconductor layer, and 
 wherein M2 is Ga, Mg, Hf, Al, Zr, Sn, or lanthanoid. 
 
     
     
       19. The semiconductor device according to  claim 15 ,
 wherein a material represented as In d M5 e Zn f O x  (d is a real number greater than 0 and less than or equal to 5, e is a real number greater than or equal to 0 and less than or equal to 3, f is a real number greater than 0 and less than or equal to 5, and x is an arbitrary positive number) is used for the second oxide semiconductor layer, and 
 wherein M5 is Ga, Mg, Hf, Al, Zr, Sn, or lanthanoid. 
 
     
     
       20. The semiconductor device according to  claim 15 ,
 wherein a material represented as In g M8 h Zn i O x  (g is a real number greater than or equal to 0 and less than or equal to 2, h is a real number greater than 0 and less than or equal to 5, i is a real number greater than or equal to 0 and less than or equal to 5, and x is an arbitrary real number) is used for the third oxide semiconductor layer, and 
 wherein M8 is Ga, Mg, Hf, Al, Zr, Sn, or lanthanoid. 
 
     
     
       21. An electronic device comprising the semiconductor device according to  claim 15 .